The present invention pertains to thin film devices and in particular thin film devices with layers exhibiting controlled microstructural morphology.
In many thin film structures and devices, the metal thin film components have a microstructure which is not symmetrical. For example, the metal thin film may have a surface roughness which is asymmetrical, i.e. the roughness is higher when measured along one direction on the film surface and less rough when measured along an orthogonal direction. The roughness asymmetry may also cause processing problems, such as excessive chemical etching of the surface in a subsequent patterning process.
In another structure the individual crystals or grains, may be longer along one direction parallel to the film surface and shorter along the orthogonal direction. It is also possible that the crystal directions within the grains may be preferentially oriented along one direction parallel to the film surface referred to in the industry as a restricted fiber texture. Other examples include asymmetrical electrical properties, optical properties, magnetic properties, stress, or mechanical properties which may be linked to the occurrence of the asymmetrical conditions set out above.
The asymmetrical microstructure originates in the deposition of the thin film. For example, deposition at an angle to the substrate plane produces an asymmetrical microstructure in molybdenum. This is set forth in the paper titled xe2x80x9cCombined Transmission Electron Microscopy and X-ray Study of the Microstructure and Texture in Sputtered Mo Filmsxe2x80x9d by O. P. Karpenko, J. C. Bilello and S. M Yalisove, Journal of Applied Physics. 76,4610 (1994).
Deposition in the presence of non-normal incidence ion bombardment produces an asymmetrical microstructure as shown for niobium (Nb) by L. S. Yu, J. M. E. Harper, J. J. Cuomo and D. A. Smith in a publication entitled xe2x80x9cAlignment of Thin Films by Glancing Angle Ion Bombardment During Depositionxe2x80x9d, Applied Physics, Lett. 47,932 (1985).
Deposition in magnetron sputtering systems which scan the substrate in a direction parallel to the target surface also produces an asymmetrical microstructure. This is shown for molybdenum (Mo) films by the authors in a paper titled xe2x80x9cIn-Plane Crystallographic Texture of bcc Metal Thin Films on Amorphous Substratesxe2x80x9d, the article is published Vol. 472, (1997) page 27, authored by J. M. E. Harper, K. P. Rodbell, E. G. Colgan and R. J. Hammond for a Proc.Mat.Res.Soc.Symp.
In some applications, asymmetrical microstructure in a thin film is desirable, for example, when the properties are improved by having a strongly aligned crystallographic texture. An example of this is in a high temperature superconductor, where the critical current density is highest along a certain crystal direction, and therefore, it is desirable to have this orientation preferentially aligned in one direction along the sample surface. Additional discussions of thin film microstructures can be found in xe2x80x9cAnisotropy in Resistivity of MoNx films at 4.2Kxe2x80x9d by S. Kominami, R. Mita and T. Nishino published in the Journal Vacuum Science Technology, B14, 3395 (1996); the xe2x80x9cEffects Of Substrates Orientation And Rotation On Internal Stresses In Sputtered Metal Filmsxe2x80x9d by D. W. Hoffmann and J. A. Thornton Journal Vacuum Science Technology 16,134 (1979); an article titled xe2x80x9cIn-Plane Texturing control of Yxe2x80x94Baxe2x80x94Cuxe2x80x94O Thin Films on Polycrystalline Substrates by Ion-Beamxe2x80x94Modified Intermediate Buffer Layers, Applied superconductivity Conference Aug. 24-28 1992, Chicago, Ill. Published in IEEE Transactions on Applied Superconductivity 3(1) (1993) pp 1510-1515.xe2x80x9d by Y. Iijima, K. Onabe, N. Futaki, N. Sadakata, O. Kohno and Y. Ikeno, xe2x80x9cProperties of Wxe2x80x94N and Moxe2x80x94N Films Prepared by Reactive Sputteringxe2x80x9d by K. K. Shih and D. B. Dove, Journal Vacuum Science Technology, A8,1359 (1990) and an article by K. L. Lin and Y. J. Ho titled xe2x80x9cDeposition and Properties of Moxe2x80x94N Filmsxe2x80x9d appearing in the Journal of Vacuum Science Technology, A13,2872 (1995).
For many applications, however, an asymmetrical microstructure in a thin film is undesirable and it is preferred to have a symmetrical microstructure and thus more uniform properties in the film.
U.S. Pat. Nos. 3,847,658; 4,000,055; 4,846,949; 5,162,933; 5,281,485; 5,421,975; 5,427,666; 5,455,197; and 5,482,785 disclose and discuss various thin film structures wherein nitrogen is incorporated into a refractory metal to impart selected properties to the film and/or the device.
The present invention pertains to a process for incorporating nitrogen and the resulting film structures achieved by incorporating nitrogen into a refractory metal to control the in-plane texture of the refractory metal layer. Incorporation of nitrogen into the refractory metal such as titanium, vanadium, chromium, niobium, molybdenum, tantalum, tungsten and alloys thereof results in a film with a microstructural morphology that is symmetrical in nature. In the context of the present invention symmetrical refers to measurements of microstructure showing substantial uniform intensity as a function of the azimuthal angle around the perpendicular to the plane of the surface. According to the present invention asymmetrical pole figure for the refractory metal/nitride film indicates that the ratio of highest to lowest intensities measured as a function of the azimuthal angle lie between 1:1 and 2:1 but no higher. Values higher than these ratios would indicate substantial azimuthal asymmetry.
According to the present invention metal thin films are preferably refractory metals consisting of one or more metals or alloys selected from the group consisting of titanium(Ti), vanadium(V), niobium(Nb), tantalum(Ta), chromium(Cr), molybdenum(Mo) and tungsten(W).
According to the present invention it has been discovered that incorporation of between 1 and 5 atomic percent nitrogen into a refractory metal thin film can overcome the asymmetric effects of the deposition environment and produce a symmetrical microstructure. The refractory metal film containing nitrogen and/or metal nitrides can be used as an underlayer, intermediate layer or in a bi-layer or multi-layer structure to obtain a symmetrical microstructure of the overall multi-layer structure.
According to the present invention refractory metals can be alloyed with other metals as long as the refractory nature of the film is maintained.
Thus in one aspect the present invention pertains to a method for modifying the microstructural morphology of a thin metal film by depositing a layer of refractory metal on a glass substrate in the presence of an inert gas to which is added an effective amount of nitrogen.
In another aspect the present invention pertains to a structure having a controlled microstructural morphology the structure being a substrate with a thin film layer comprising a refractory metal containing from 1 to 5% nitrogen by atomic fraction deposited in the substrate.